Thermoplastic copolyetheresters can be described as copolymers consisting of recurring long chain and short chain ester units joined through ester linkages. The long chain ester units are formed by esterification of a dicarboxylic acid with a polyether glycol, the short chain ester units by esterification of a dicarboxylic acid with a low molecular weight diol.
Relatively soft copolyetheresters (hardness &lt;40D) based on poly(tetramethylene oxide) glycol (PTMEG) in which butylene terephthalate is the principal short chain ester unit are known. These polymers have good physical properties but do not process as well as desired in high-speed injection molding or extrusion operations due to their rate of hardening. One can improve hardening rates of the polymer by using during its preparation higher molecular weight PTMEG but this results in impairment of low temperature properties of the polymer. Replacing PTMEG with poly(propylene oxide) glycol (PPG) or poly(ethylene oxide) glycol does not yield acceptable polymer products because of the low reactivity of the former and the excessive water sensitivity of the latter. In the case of PPG, phasing during the melt condensation (inhomogeneity of the polymerization mass because of incompatibility of short and long chain ester units) is also a problem. Phasing can lead to nonrandomized polymer compositions having inferior physical properties. It is desirable to obtain a copolyetherester having both good physical properties and processing characteristics.